1. Field of the Invention
The present invention relates to a power circuit using a chemical cell and more particularly to the power circuit that can be suitably used when power is supplied to a load whose power consumption changes intermittently, for example, to a power source of a power amplifier for transmission of a radio wave embedded in a portable cellular phone and to a method for controlling the power circuit and an electronic device using the above power circuit.
The present application claims priority of Japanese Patent Application No.2000-318310 filed on Oct. 18,2000, which is hereby incorporated by reference.
2. Description of the Related Art
When a power circuit using a chemical cell is connected to a load whose power consumption changes intermittently, since an internal impedance of the chemical cell is comparatively large, a phenomenon occurs in which a voltage of the chemical cell drops instantaneously at a same time when a load current increases instantaneously. To solve this problem, conventionally, a capacitor having a comparatively low internal impedance is connected to the chemical cell in parallel. This allows a combined impedance whose level is lower than that of the internal impedance of the chemical cell to be formed and therefore even when the load whose power consumption changes intermittently is connected, an instantaneous voltage drop rate in voltage decreases compared with a case in which the chemical cell is singly used.
The conventional power circuit of this type, as shown in FIG. 5, includes a chemical cell 1 and a capacitor 2 connected to the chemical cell 1 in parallel and a load L connected to the chemical cell 1 and the capacitor 2. The chemical cell 1 is made up of, for example, a secondary cell such as a nickel-cadmium cell, nickel-hydrogen cell, or lithium ion cell, or an alkaline primary cell. Such the chemical cell 1 stores a predetermined amount of energy, produces electromotive force (that is, voltage V1) based on the stored energy and supplies it to the load L. The chemical cell 1 has an internal impedance 1a. The capacitor 2 is made up of, for example, an electrical double layer capacitor and is charged by the voltage V1 of the chemical cell 1, thus accumulating electric power, and then feeds the accumulated power to the load L. The capacitor 2 has an internal impedance 2a. The load L is, for example, a power amplifier for transmission of radio waves embedded in portable cellular phone or a like, whose power consumption changes intermittently and through which a pulse-like load current IL flows.
FIG. 6 is a timing chart explaining operations of the conventional power circuit of FIG. 5, in which a current or voltage is plotted as ordinate and time is plotted as abscissa. Operations of the power circuit of FIG. 5 will be described by referring to FIG. 6.
At a time t1, the load current IL increases instantaneously and the voltage V1 of the chemical cell 1 drops from a voltage level Va to a voltage level Vb. At this point, an internal impedance Z of the power circuit is given by:
Z=R1xc3x97R2/(R1+R2) 
where R1 denotes a value of the internal impedance 1a and R2 denotes a value of the internal impedance 2a. The internal impedance Z is smaller than the value R1 of the internal impedance 1a. Therefore, a drop rate of the voltage Vb is lower than that of a voltage Vc occurring when the power circuit is made up of only the chemical cell 1. At a time t2, the pulse-like load current IL decreases instantaneously and the voltage V1 returns from the voltage level Vb to the voltage level Va. A voltage V2 of the capacitor 2 changes in the same manner as in the voltage V1.
Thus, when the capacitor 2 is connected to the chemical cell 1 in parallel, since the drop rate of the voltage Vb is lower than that of the voltage Vc, time during which the chemical cell 1 can be used per one time charging is made longer compared with the case in which the power circuit is made up of only the chemical cell 1. Moreover, when the chemical cell 1 is constructed of the alkaline primary cell having a comparatively high internal impedance, a life of the chemical cell 1 is made longer when compared with the case in which the power circuit is made up of only the chemical cell 1.
However, the above conventional power circuit has a following problem.
That is, in an electronic device having the power circuit shown in FIG. 5, since judgement on a residual capacity of the chemical cell 1 is made based on the drop in the voltage V1, the chemical cell 1 is judged to have no residual capacity even by the instantaneous drop in the voltage V1 in some cases. However, when the chemical cell 1 is constructed of, for example, the alkaline primary cell, even if the chemical cell 1 is judged to have no residual capacity in an electronic device, in some cases, the chemical cell 1 can be used in another electronic device. This phenomenon shows that the chemical cell 1 has not run out of its capacity completely. As a result, the chemical cell 1 is judged to have gotten to an end of its life in a state where a depth of discharge (a ratio of discharged capacity to a rated capacity) of the chemical cell 1 is still shallow, thus causing a decrease in use efficiency of the energy of the chemical cell 1.
On the other hand, when the chemical cell 1 is made up of, for example, the secondary cell such as the nickel-cadmium cell, nickel-hydrogen cell, lithium ion cell, or a like, there is a problem that, if the chemical cell 1 has not run out of the capacity completely, time during which the chemical cell 1 can be used per one time charging becomes extremely shorter than the usable time that the capacity of the original chemical cell 1 can provide. Moreover, when the capacitor 2 is connected to the chemical cell 1 in parallel, if the capacitor 2 has not been charged, since a large inrush current flows from the chemical cell 1 into the capacitor 2, burning of a wiring pattern used to connect the chemical cell 1 to the capacitor 2 and/or degradation of the chemical cell 1 and the capacitor 2 occur in some cases. In particular, when the chemical cell 1 is made up of the lithium ion secondary cell in which a current control circuit is embedded, a failure occurs in the current control circuit.
In view of the above, it is an object of the present invention to provide a power circuit capable of improving use efficiency of energy in a chemical cell and of preventing an inrush current from flowing from the chemical cell into a capacitor even when the capacitor having not yet been charged is connected to the chemical cell in parallel and a method for controlling the above power circuit and an electronic device using the power circuit.
According to a first aspect of the present invention, there is provided a power circuit including:
a chemical cell used to store a predetermined amount of energy, to produce electromotive force based on the energy and to feed the electromotive force to a load;
a capacitor which is charged by the electromotive force produced by the chemical cell and accumulates electric power and applies the accumulated electric power to the load;
a control section; and
wherein the control section, when a voltage of the capacitor is higher than a first reference value and when a load current flowing through the load is smaller than a second reference value, applies a voltage produced by the chemical cell to the capacitor from the chemical cell to charge the capacitor and, at a same time, feeds the electromotive force produced by the chemical cell and the accumulated electric power in the capacitor to the load and, when the load current is larger than the second reference value, feeds only the accumulated electric power in the capacitor to the load and, when a voltage of the capacitor is lower than the first reference value and the load current flowing through the load is smaller than the second reference value, applies a current whose level is limited to a predetermined level to the capacitor from the chemical cell to charge the capacitor and feeds the electromotive force produced by the chemical cell to the load.
In the foregoing, a preferable mode is one wherein the control section includes:
a capacitor voltage detecting unit which compares a voltage of the capacitor with the first reference value and, when the voltage of the capacitor is lower than the first reference value, outputs a first detection signal being in an active mode and, when the voltage of the capacitor is higher than the first reference value, outputs the first detection signal being in a non-active mode;
a load current detecting unit which detects the load current and, when the load current is higher than the second reference value, outputs a second detection signal being in an active mode and, when the load current is smaller than the second reference value, outputs the second detection signal being in a non-active mode; and
an electric power feeding and charging unit which feeds, when the first detection signal is in the non-active mode and the second detection signal is in the non-active mode, a voltage produced by the chemical cell from the chemical cell to the capacitor to charge the capacitor and, at a same time, feeds the electromotive force produced by the chemical cell and the accumulated electric power in the capacitor to the load and, when the second detection signal is in the active mode, feeds only the accumulated electric power in the capacitor to the capacitor and, when the first detection signal is in the active mode and the second detection signal is in the non-active mode, feeds a current whose level is limited to a predetermined level to the capacitor from the chemical cell to charge the capacitor and, at a same time, applies the electromotive force produced by the chemical cell to the load.
Also, a preferable mode is one wherein the capacitor is made up of an electrical double layer capacitor having an internal impedance being lower than an internal impedance of the chemical cell.
Furthermore, a preferable mode is one wherein the capacitor, while only the accumulated electric power in the capacitor is fed to the load, stores electric power exceeding power consumed by the load when the second detection signal is in the active mode.
According to a second aspect of the present invention, there is provided a method for controlling a power circuit having a chemical cell used to store a predetermined amount of energy, to produce electromotive force based on the energy and to feed the electromotive force to a load, a capacitor which is charged by the electromotive force produced by the chemical cell and accumulates electric power and applies the accumulated electric power to the load and a control section, the method including:
a step of applying, by using the control section, when a voltage of the capacitor is higher than a first reference value and a load current flowing through the load is smaller than a second reference value, a voltage produced by the chemical cell to the capacitor from the chemical cell to charge the capacitor, feeding, at a same time, the electromotive force produced by the chemical cell and the accumulated electric power in the capacitor to the load, feeding only the accumulated electric power in the capacitor to the load when the load current is larger than the second reference value and, applying, when a voltage of the capacitor is lower than the first reference value and when the load current flowing through the load is smaller than the second reference value, a current whose level is limited to a predetermined level to the capacitor from the chemical cell to charge the capacitor and feeding the electromotive force produced by the chemical cell to the load.
According to a third aspect of the present invention, there is provided a method for controlling a power circuit having a chemical cell used to store a predetermined amount of energy, to produce electromotive force based on the energy and to feed the electromotive force to a load, a capacitor which is charged by the electromotive force produced by the chemical cell and accumulates electric power and applies the accumulated electric power to the load and a control section, the method including:
a step of mounting a capacitor voltage detecting unit, a load current detecting unit, and an electric power feeding and charging unit to the control unit;
a capacitor voltage detecting step of, by using the capacitor voltage detecting unit, detecting a voltage of the capacitor, comparing the detected voltage with the first reference value, outputting, when the voltage of the capacitor is lower than the first reference value, a first detection signal being in an active mode and outputting, when the voltage of the capacitor is higher than the first reference value, the first detection signal being in a non-active mode;
a load current detecting step of, by using the load current detecting unit, detecting the load current flowing through the load, comparing the detected load current with the second reference value and outputting, when the load current is higher than the second reference value, a second detection signal being in an active mode and outputting, when the load current is smaller than the second reference value, the second detection signal being in a non-active mode.
a first electric power feeding and charging step of, by using the electric power feeding and charging unit, feeding, when the first detection signal is in the non-active mode and the second detection signal is in the non-active mode, a voltage produced by the chemical cell from the chemical cell to the capacitor to charge the capacitor and feeding, at a same time, electromotive force produced by the chemical cell and electric power accumulated in the capacitor to the load;
an electric power feeding processing step of, by using the electric power feeding and charging unit, feeding, when the second detection signal is in the active mode, only electric power accumulated in the capacitor to the capacitor; and
a second electric power feeding and charging step of, by using the electric power feeding and charging unit, feeding, when the first detection signal is in the active mode and the second detection signal is in the non-active mode, a current whose level is limited to a predetermined level to the capacitor from the chemical cell to charge the capacitor and applying, at a same time, electromotive force produced by the chemical cell to the load.
According to a fourth aspect of the present invention, there is provided an electronic device made up of a power circuit stated above.
According to a fifth aspect of the present invention, there is provided an electronic device using a method for controlling a power circuit stated above.
With the above configurations, when the load current increases instantaneously, only the accumulated electric power in the capacitor is fed to the load and therefore no drop in the voltage of the chemical cell occurs. Therefore, even if life of the chemical cell is judged based on a drop in the voltage, the chemical cell is not judged to have gotten to an end of its life in a state where a depth of discharge of the chemical cell is still shallow and, as a result, use efficiency of the energy of the chemical cell is improved. Moreover, when the voltage of the capacitor is lower than a reference voltage, since the current whose level has been controlled and limited to a predetermined level is fed from the chemical cell and the capacitor is charged, even while the capacitor has not yet been charged, no inrush current flows from the chemical cell into the capacitor, thus preventing burning of a wiring pattern connecting the chemical cell to the capacitor and/or degradation of the chemical cell and the capacitor.